All aircraft include movable control surfaces for directional control in flight. Such control surfaces can include ailerons for roll control, elevators for pitch control, and rudders for yaw control. In addition, most conventional jet transport aircraft typically include leading edge slats and trailing edge flaps on the wings. These devices can be used to generate high lift during takeoff and landing when the aircraft is traveling at relatively low air speeds.
Federal aviation regulations (FARs) impose airworthiness standards on lift and drag devices for transport category aircraft. For example, FAR §25.697 requires that such devices (e.g., trailing edge flaps) must maintain selected positions (e.g., extended positions) without further attention by the pilot. This requirement applies at all times during flight. Thus, lift and drag devices must be able to maintain extended positions even in the unlikely event of a general failure of the aircraft's power system.
Trailing edge flaps (“flaps”) on jet transport aircraft typically deploy aft of the wing and downward to increase wing area and camber. The flaps are typically powered by a drive system having a drive shaft that extends longitudinally inside the wing and is coupled to a central power drive unit. The drive shaft for each wing is connected by a system of gears to a series of ball screws and linear actuators distributed along the length of the wing adjacent to the flaps. Rotation of the drive shaft in a first direction causes the ball screws to rotate in a corresponding direction, thereby extending the flaps on the wing. Similarly, counter rotation of the drive shaft causes the ball screws to counter-rotate, thereby retracting the flaps. Flap drive systems are mechanically interconnected to provide wing-to-wing symmetry of the trailing edge flaps on both wings. Such wing-to-wing symmetry, or equivalent, is required by the current FARs. These conventional drive systems, however, can be very heavy and costly.
Hydraulic drive systems with linear actuators have also been used for flap drive systems. For safety purposes, these hydraulic flap drive systems are typically designed to include built-in backup or redundant systems. Accordingly, the hydraulic flap drive systems are powered by two hydraulic systems and utilize twice as many linear actuators as are required to handle the system loads. The resulting hydraulic flap drive systems tend to weigh more and cost more than the drive systems using the drive shafts and gears.